There are various elongated non-magnetic, non-conductive items, such as ducts, cable-carrying conduits or plastic carriers of liquids and gases, mainly natural gas pipelines, that are buried a short distance below the surface of the earth. These items, once buried, are difficult to detect without having an exact plan of the area in which they are buried, with reference to existing structures or points. After many years, such reference structures or points may have been destroyed or the plans may be lost and it then is almost impossible to locate the buried items if required, as for replacement or repair. The problem is not particularly present with magnetic items or items that can carry an electrical current. There are many systems available for the detection of such buried items.
There are also different methods in place for the detection of non-magnetic or non-conductive pipes or cables, including those described in U.S. Pat. Nos. 5,036,210; 5,051,034; and RE34,701 of Goodman or the U.S. Pat. Nos. 5,006,806; 5,017,873 and 5,114,517 of Rippingdale et al. The inventions of Goodman and Rippingdale et al provide the pipe, usually of a plastic material, with strips, either continuous or discontinuous, of a permanently magnetized material so that the magnetic field associated with such strips can be detected by a suitable detector, such as a gradiometer, moving across the surface of the earth in the vicinity of the buried item. As an alternative to providing magnetic strips on the buried item Goodman, for example, suggests extruding a pipe with magnetic particles mixed into the plastic material used for the pipe prior to extrusion of the pipe so that the magnetic particles are uniformly dispersed throughout the pipe after it has cured. The pipe can then be subjected to a high strength magnetic field so that the particles become permanently magnetized and thereafter have a magnetic field permanently associated therewith.
The methods and products provided by Goodman and Rippingdale et al will work adequately, but the cost of manufacture is high and in some cases prohibitive, as in the case of coiled plastic pipes and conduits, which are of relatively small diameter. There is therefore a need for an improved method of magnetic detection, one which allows for new manufacture of pipes, ducts and conduits ready for burial and later detection, and which also allows for their preparation after manufacture with a magnetically detectable material associated therewith.